1. Field of the Invention
The present invention relates to a method for manufacturing an optical film-forming pressure-sensitive adhesive layer using an aqueous dispersion-type pressure-sensitive adhesive. The present invention also relates to an optical film-forming pressure-sensitive adhesive layer obtained by the manufacturing method and to a pressure-sensitive adhesive-type optical film including an optical film and the pressure-sensitive adhesive layer placed thereon. The present invention also relates to an image display device such as a liquid crystal display device, an organic electroluminescence (EL) display device, a cathode-ray tube (CRT), or a plasma display panel (PDP) produced using the pressure-sensitive adhesive-type optical film and to a part used together with an image display device, such as a front face plate, produced using the pressure-sensitive adhesive-type optical film. Examples of the optical film that may be used include a polarizing plate, a retardation plate, an optical compensation film, a brightness enhancement film, a surface treatment film such as an anti-reflection film, and a laminate of any combination thereof.
2. Description of the Related Art
Liquid crystal display devices, organic EL display devices, etc. have an image-forming mechanism including polarizing elements as essential components. For example, therefore, in a liquid crystal display device, polarizing elements are essentially placed on both sides of a liquid crystal cell, and generally, polarizing plates are attached as the polarizing elements. Besides polarizing plates, various optical elements have been used in display panels such as liquid crystal panels and organic EL panels for improving display quality. Front face plates are also used to protect image display devices such as liquid crystal display devices, organic EL display devices, CRTs, and PDPs or to provide a high-grade appearance or a differentiated design. Examples of parts used in image display devices such as liquid crystal display devices and organic EL display devices or parts used together with image display devices, such as front face plates, include retardation plates for preventing discoloration, viewing angle-widening films for improving the viewing angle of liquid crystal displays, brightness enhancement films for increasing the contrast of displays, and surface treatment films such as hard-coat films for use in imparting scratch resistance to surfaces, antiglare treatment films for preventing glare on image display devices, and anti-reflection films such as anti-reflective films and low-reflective films. These films are generically called optical films.
When such optical films are bonded to a display panel such as a liquid crystal cell or an organic EL panel or bonded to a front face plate, a pressure-sensitive adhesive is generally used. In the process of bonding an optical film to a display panel such as a liquid crystal cell or an organic EL panel or to a front face plate or bonding optical films together, a pressure-sensitive adhesive is generally used to bond the materials together so that optical loss can be reduced. In such a case, a pressure-sensitive adhesive-type optical film including an optical film and a pressure-sensitive adhesive layer previously formed on one side of the optical film is generally used, because it has some advantages such as no need for a drying process to fix the optical film.
Conventionally, organic solvent-type pressure-sensitive adhesives have been dominantly used for forming the pressure-sensitive adhesive layer of such a pressure-sensitive adhesive-type optical film, because of their high durability in heated or heated and humid environments. Organic solvent-type pressure-sensitive adhesives that are generally used include acryl-based pressure-sensitive adhesives, which are colorless and transparent and have good tackiness to display panels such as liquid crystal cells and organic EL panels or to front face plates. It has been proposed that the pressure-sensitive adhesive layer should have a controlled surface state from various points of view.
The organic solvent-type pressure-sensitive adhesives are solutions with low solid concentrations (generally about 10 to 15% by weight) and therefore are more likely to form coatings with reduced surface smoothness due to uneven drying or uneven air flow, which can affect the visibility of pressure-sensitive adhesive layers made from such solutions. For example, it is proposed that the thickness and surface roughness of a pressure-sensitive adhesive layer formed by coating should be controlled so that the pressure-sensitive adhesive layer can have reduced irregularities and improved visibility (JP-A-2008-302580). Unfortunately, the disclosure in JP-A-2008-302580 has a problem in which the improvements involve a reduction in productivity, such as a reduction in drying temperature or coating speed.
In recent years, solvent-free pressure-sensitive adhesives, which are produced with no organic solvent, have been developed aggressively in view of a reduction in global environmental loading or an improvement in process stability. Proposed solvent-free pressure-sensitive adhesives include aqueous dispersion-type pressure-sensitive adhesives containing a pressure-sensitive adhesive component dispersed in water used as a dispersion medium. For example, it is proposed that the surface tension of an aqueous dispersion-type pressure-sensitive adhesive should be controlled in order to form a smooth pressure-sensitive adhesive layer (JP-A-2005-320487). Although the method of JP-A-2005-320487 makes it possible to make a smooth pressure-sensitive adhesive layer from an aqueous dispersion-type pressure-sensitive adhesive, optical film-forming pressure-sensitive adhesive layers have been required to have further improved smoothness and higher visibility. There is also proposed an aqueous dispersion-type acryl-based pressure-sensitive adhesive composition containing an acryl-based copolymer emulsion with controlled solid concentration, average particle size, and viscosity (JP-A-08-218047). JP-A-08-218047 discloses that spew- or shrink-free smooth coating surfaces required for Japanese paper tape applications can be obtained. Unfortunately, the level of smoothness disclosed in JP-A-08-218047 is not enough for optical film applications, and the disclosure in JP-A-08-218047 does not satisfy the smoothness required of optical film-forming pressure-sensitive adhesive layers.